(CNN) -- Tioman is a verdant jewel of an island, with a near legendry status among divers. You might not have heard of it, but you might have seen it.

Tioman's beaches were used as a filming location for the 1958 classic movie "South Pacific". The grandfather of scuba Jacques Cousteau also came here, rating its reefs and in the 1970s Time Magazine voted it among the top ten most beautiful islands in the world.

But now Tioman has reached a critical crossroads, according to those who have devoted their life to preserving its unique marine and jungle habitat.

Katie Yewdall runs Blue Ventures on the island, an eco-tourism company that offers volunteers the chance to leave the island in a better condition than they found it.

Tioman is at a point now where it can become very commercial or go towards more responsible tourism.--Katie Yewdall, Blue Ventures

"Tioman is part of the coral triangle which extends from the Philippines, down to Papua New Guinea and across to this coast of Malaysia, where the coral reefs are some of the most highly diverse in the world," she says.

"There's something like 700 genera of coral in this area. Tourism has been here for thirty years but it's definitely growing. It's kind of at the point now where it can go in two directions it can either go down the very commercial road and become very much like neighboring countries like Thailand or it can choose to take a different direction and go towards more responsible tourism."

You only have to visit an island like Koh Samui in Thailand to realize why environmentalists on Tioman are so worried. Samui has been heavily developed with shopping malls, hundreds of hotels, vacation homes and heavy traffic on its tiny roads.

In 2009, Samui had 650,000 visitors last year, compared to less than 200,000 last year on Tioman.

But now the Malaysian federal government and local developers want to build a new airport on Tioman, superseding the tiny airstrip that currently allows only small propeller planes to perform a hair-raising landing between the mountains and the sea.

The new airport would allow much bigger jets to land and would open up the possibility that budget carriers could open routes here.

Local leaders, like Kamarulzaman bin Ismail say it's vital for the economy, but they are convinced they can still preserve Tioman's unique environment. He knows the reason tourists come here is for the pristine environment; ruining that could deter future visitors.

Dan Pedraza is a volunteer with Blue Ventures and is convinced that tourism and environmentalism can co-exist. His own personal journey represents just the sort of change in thinking he feels is necessary on Tioman. Dan used to be a Formula 1 engineer but opted out of the "Rat Race" to lead a greener life.

"I think that it doesn't have to necessarily have a negative impact on a place and secondly tourists can actually bring something positive with them, so they don't necessarily have to come and take away but they can bring with them something," he says.

Tioman is actually a designated marine park, which should mean no building into the sea, no fishing within 2 km of the shore and tight controls on development. But the rules have failed to stop a controversial marina from being built.

Abdul Manap Abdullah from the Marine Park Department says: "For the environment we worry about it but we cannot do anything because this is on other authorities, if they allow it, we cannot do anything."

When I ask him how a marina was allowed when such strict rules prohibit building into the sea, he becomes cagey. He knows the federal and state governments and powerful local Royal family backed the scheme, despite its apparent incompatibility with the protection offered to the marine park.

But the rest of Tioman is barely touched, basking in the crystal waters of the South China Sea. The interior is mountainous and covered by impenetrable rainforest. It's rumored only four people on the island know how to climb to its highest peak, a trek that takes three days. Jungle giants tower on the steep hillsides, the sort of trees that were felled hundreds of years ago on other islands, but have survived here in part because, at times, this island has been depopulated by marauding pirates or disease.

Now the new challenge is tourism, which if it isn't managed carefully could irrevocably change this tropical Eden into a forgettable mass-market holiday destination.